1. Field of the Invention
The invention relates to a titanium-containing catalyst with high catalytic activity for the production of polyester, whereby the catalytic activity is not reduced by the water formed during esterification. The invention also relates to a process for the production of thermostable, perfectly colored polyesters with an ether-content as low as possible by esterification or transesterification and subsequent polycondensation in the presence of this titanium-containing catalyst.
2. Summary of the Related Art
Generally, polyesters are produced by reaction of a diol with a dicarboxylic acid or a lower alkyl ester of a dicarboxylic acid, e.g., the dimethylester. At first, the corresponding dicarboxylic acid diester is formed, which is then polycondensed at increasing temperature and reduced pressure, whereby diol and water are split off. Both reaction steps need catalysts or are at least accelerated by them. Titanium compounds are suitable catalysts for the esterification, compounds of Mn, Co or Zn for the transesterification, and compounds of Sb, Ti, Pb, Ge, Zn or Sn for the polycondensation, whereby the compounds are in general oxides, alcoholates, acetates, or carboxylates. The amount of metal in the catalyst ranges mostly between 20 and 500 ppm, based on polyester.
Among these catalysts the titanium compounds are the most effective and the most widely usable because they are absolutely non-poisonous and can be used for the esterification or transesterification as well as for the polycondensation. Only for polyethylene terephthalate (PET) is the use of co-catalysts necessary to avoid discoloring. The use of the titanium as catalyst takes place mainly in the form of alcoholates, but the use of titanium salts is also described. However, the frequently used titanium alcoholates are known to be hydrolyzed during the esterification stage by the water formed there, and by this to be catalytically inactivated, which makes a further addition of catalyst in the polycondensation and generally high amounts of catalyst necessary.
There are no definite reports on the catalytic activity of TiO.sub.2, but in general it is assumed to be to a large extent catalytically inactive. TiO.sub.2 is used, in addition to usual catalysts, in large quantities, mostly in the anatase form in the PET fiber production as a delustering agent and as a white pigment. The usual concentrations are 0.1 to 3 wt. % TiO.sub.2 /PET. In the process of U.S. Pat. No. 2,906,737, TiO.sub.2 is used in the rutile form in quantities of 0.01 to 5 wt. % TiO.sub.2 as an esterification and polycondensation catalyst. The esterification time of more than 7 hours is a distinct drawback of this method, however. According to U.S. Pat. No. 3,056,817, a titanium dioxide gel obtained by alkaline precipitation and consisting of about 5 wt. % TiO.sub.2 and 95 wt. % water is used in quantities of 0.01 to 10 wt. %, based on dicarboxylic acid as catalyst for the esterification stage. However, the catalytic activity of titanium dioxides produced in this way is very low. In the process of U.S. Pat. No. 3,463,742, freshly precipitated titanium dioxide hydrate is freed from residual water, dispersed in butanol, and the suspension in butanol (with 2 to 20 wt. % Ti) used as catalyst for the transesterification and polycondensation of polyester. Thereafter, however, the butanol contaminates the reaction vapors, which should be recycled within the polyester process. The necessity to freshly produce the catalyst immediately before its use, i.e., to have to construct a catalyst plant in addition to the polyester plant, is disadvantageous in both cases.
The use of various titanium salts, e.g., lithium or sodium titanate is disclosed in U.S. Pat. No. 3,965,071. As polyester with severe discoloration and very high diethylene glycol content is otherwise produced, the titanium catalyst must be completely deactivated after the esterification by addition of phosphorous compounds, and the polycondensation continued in the presence of other catalysts like antimony compounds or the very expensive germanium compounds. U.S. Pat. No. 4,365,054 discloses alkali titanates of the formula (M.sub.2 O)(TiO.sub.2).sub.n, wherein n=0.05 to 25 and M is an alkali metal, as catalyst of esterification as well as catalyst of polycondensation for the production of polyester. The highest catalytic activity is achieved with n=3 to 5, but the ether content of the polyester is relatively high, for example 1.33 wt. %. for lithium titanate and 2.15 wt. % for sodium titanate. The titanates are obtained by melting together alkali carbonate and TiO.sub.2. The crystallite size and the particle size are not disclosed.